Is good to use if you have alot of bright saturated colours.
If “Perceptual” is not the option to use, the “Relative” will produce more accurate prints.
Below is an extract from the LR3.2 help library:
Colours that are in-gamut may change as out-of-gamut colors are shifted to reproducible colours.
Perceptual rendering is a good choice when your image has many out-of gamut colors.
The Relative option preserves more of the original color and is a good choice when you have few out-of-gamut colors.